Suberin: the biopolyester at the frontier of plants

AtMYB41 acts as a dual-function transcription factor that regulates the formation of lipids in an organ- and development-dependent manner

The plant cuticle controls non-stomatal water loss and can serve as a barrier against biotic agents, whereas the heteropolymer suberin and its associated waxes are deposited constitutively at specific cell wall locations. While several transcription factors controlling cuticle formation have been identified, those involved in the transcriptional regulation of suberin biosynthesis remain poorly characterized. The major goal of this study was to further analyse the function of the R2R3-Myeloblastosis (MYB) transcription factor AtMYB41 in formation of the cuticle, suberin, and suberin-associated waxes throughout plant development. For functional analysis, the organ-specific expression pattern of AtMYB41 was analysed and Atmyb41ge alleles were generated using the CRISPR/Cas9 system. These were investigated for root growth and water permeability upon stress. In addition, the fatty acid, wax, cutin, and suberin monomer composition of different organs was evaluated by gas chromatography. The characterization of Atmyb41ge mutants revealed that AtMYB41 negatively regulates the production of cuticular lipids and fatty acid biosynthesis in leaves and seeds, respectively. Remarkably, biochemical analyses indicate that AtMYB41 also positively regulates the formation of cuticular waxes in stems of Arabidopsis thaliana. Overall, these results suggest that the AtMYB41 acts as a negative regulator of cuticle and fatty acid biosynthesis in leaves and seeds, respectively, but also as a positive regulator of wax production in A. thaliana stems.

The GPAT4/6/8 clade functions in Arabidopsis root suberization nonredundantly with the GPAT5/7 clade required for suberin lamellae

Lipid polymers such as cutin and suberin strengthen the diffusion barrier properties of the cell wall in specific cell types and are essential for water relations, mineral nutrition, and stress protection in plants. Land plant-specific glycerol-3-phosphate acyltransferases (GPATs) of different clades are central players in cutin and suberin monomer biosynthesis. Here, we show that the GPAT4/6/8 clade in Arabidopsis thaliana, which is known to mediate cutin formation, is also required for developmentally regulated root suberization, in addition to the established roles of GPAT5/7 in suberization. The GPAT5/7 clade is mainly required for abscisic acid-regulated suberization. In addition, the GPAT5/7 clade is crucial for the formation of the typical lamellated suberin ultrastructure observed by transmission electron microscopy, as distinct amorphous globular polyester structures were deposited in the apoplast of the gpat5 gpat7 double mutant, in contrast to the thinner but still lamellated suberin deposition in the gpat4 gpat6 gpat8 triple mutant. Site-directed mutagenesis revealed that the intrinsic phosphatase activity of GPAT4, GPAT6, and GPAT8, which leads to monoacylglycerol biosynthesis, contributes to suberin formation. GPAT5/7 lack an active phosphatase domain and the amorphous globular polyester structure observed in the gpat5 gpat7 double mutant was partially reverted by treatment with a phosphatase inhibitor or the expression of phosphatase-dead variants of GPAT4/6/8. Thus, GPATs that lack an active phosphatase domain synthetize lysophosphatidic acids that might play a role in the formation of the lamellated structure of suberin. GPATs with active and nonactive phosphatase domains appear to have nonredundant functions and must cooperate to achieve the efficient biosynthesis of correctly structured suberin.

Exogenous abscisic acid induces the formation of a suberized barrier to radial oxygen loss in adventitious roots of barley (Hordeum vulgare)

BACKGROUND AND AIMS

Internal root aeration is essential for root growth in waterlogged conditions. Aerenchyma provides a path for oxygen to diffuse to the roots. In most wetland species, including rice, a barrier to radial oxygen loss (ROL) allows more of the oxygen to diffuse to the root tip, enabling root growth into anoxic soil. Most dryland crops, including barley, do not form a root ROL barrier. We previously found that abscisic acid (ABA) signalling is involved in the induction of ROL barrier formation in rice during waterlogging. Although rice typically does not form a tight ROL barrier in roots in aerated conditions, an ROL barrier with suberized exodermis was induced by application of exogenous ABA. Therefore, we hypothesized that ABA application could also trigger root ROL barrier formation with hypodermal suberization in barley.

METHODS

Formation of an ROL barrier was examined in roots in different exogenous ABA concentrations and at different time points using cylindrical electrodes and Methylene Blue staining. Additionally, we evaluated root porosity and observed suberin and lignin modification. Suberin, lignin and Casparian strips in the cell walls were observed by histochemical staining. We also evaluated the permeability of the apoplast to a tracer.

KEY RESULTS

Application of ABA induced suberization and ROL barrier formation in the adventitious roots of barley. The hypodermis also formed lignin-containing Casparian strips and a barrier to the infiltration of an apoplastic tracer (periodic acid). However, ABA application did not affect root porosity.

CONCLUSIONS

Our results show that in artificial conditions, barley can induce the formation of ROL and apoplastic barriers in the outer part of roots if ABA is applied exogenously. The difference in ROL barrier inducibility between barley (an upland species) and rice (a wetland species) might be attributable to differences in ABA signalling in roots in response to waterlogging conditions.

Bark water uptake through lenticels increases stem hydration and contributes to stem swelling

Foliar water uptake can recharge water storage tissue and enable greater hydration than through access to soil water alone; however, few studies have explored the role of the bark in facilitating water uptake. We investigated pathways and dynamics of bark water uptake (BWU) in stems of the mangrove Avicennia marina. We provide novel evidence that specific entry points control dynamics of water uptake through the outer bark surface. Furthermore, using a fluorescent symplastic tracer dye we provide the first evidence that lenticels on the outer bark surface facilitate BWU, thus increasing stem water content by up to 3.7%. X-ray micro-computed tomography showed that BWU was sufficient to cause measurable swelling of stem tissue layers increasing whole stem cross-sectional area by 0.83 mm2 or 2.8%, implicating it as a contributor to the diel patterns of water storage recharge that buffer xylem water potential and maintain hydration of living tissue.

Determination of De Novo Suberin-Lignin Ferulate Deposition in Xylem Tissue Upon Vascular Pathogen Attack

Plant vascular pathogens use different ways to reach the xylem vessels and cause devastating diseases in plants. Resistant and tolerant plants have evolved various defense mechanisms against vascular pathogens. Inducible physico-chemical structures, such as the formation of tyloses and wall reinforcements with phenolic polymers, are very effective barriers that confine the pathogen and prevent colonization. Here, we use a combination of classical histochemistry along with bright-field and fluorescence microscopy and two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy to visualize and characterize wall reinforcements containing phenolic wall polymers, namely, lignin, ferulates, and suberin, which occur in different xylem vasculature in response to pathogen attack.

The evolutionary innovation of root suberin lamellae contributed to the rise of seed plants

Seed plants overtook ferns to become the dominant plant group during the late Carboniferous, a period in which the climate became colder and dryer1,2. However, the specific innovations driving the success of seed plants are not clear. Here we report that the appearance of suberin lamellae (SL) contributed to the rise of seed plants. We show that the Casparian strip and SL vascular barriers evolved at different times, with the former originating in the most recent common ancestor (MRCA) of vascular plants and the latter in the MRCA of seed plants. Our results further suggest that most of the genes required for suberin formation arose through gene duplication in the MRCA of seed plants. We show that the appearance of the SL in the MRCA of seed plants enhanced drought tolerance through preventing water loss from the stele. We hypothesize that SL provide a decisive selective advantage over ferns in arid environments, resulting in the decline of ferns and the rise of gymnosperms. This study provides insights into the evolutionary success of seed plants and has implications for engineering drought-tolerant crops or fern varieties.

Neither lysigenous nor just oil: Demystifying myrtaceous secretory cavities

PREMISE

Leaf subepidermal secretory cavities are a notable trait in Myrtaceae, but their formation is still controversial because of the lack of consensus on their ontogeny among authors. Knowledge about the compounds present in these cavities has grown over the last few years, demonstrating that terpenoid-rich oils are not their unique content. These two points are the focus of this study on the ontogeny, structure, and contents of secretory cavities in neotropical Myrtaceae.

METHODS

We used histochemical tests and Raman analysis to verify the basic chemical composition of the cavity contents of nine species. We studied the ontogeny of glands in one species, comparing aldehyde-fixed tissues and fresh sections mounted in an inert medium.

RESULTS

We observed schizogenous development and appearance of the secretory cavities and found that sample processing may induce cell breakdown, which can be misinterpreted as lysigeny. The content of these cavities contains putative terpenes, resins, carbonyl groups, and flavonoids.

CONCLUSIONS

Our findings support the hypothesis that the lysigenous appearance of the oil glands is a technical artifact. These tissue distortions must be considered when interpreting the development of this type of secretory structure. Moreover, the basic analyses of chemical constituents show for the first time that the glands of neotropical Myrtaceae are potential reservoirs of some compounds such as flavonoids previously reported as novelties for a few other myrtaceous species. Because some of them are non-lipid compounds, the idea that the glands are just oil repositories is no longer applicable.

Comparative transcriptome provides insights into gene regulation network associated with the resistance to Fusarium wilt in grafted wax gourd Benincasa hispida

Introduction

Wilt is a soil-borne disease caused by Fusarium that has become a serious threat to wax gourd production. Disease-resistant graft wax gourds are an effective treatment for Fusarium wilt. However, there are few reports on the defense mechanism of graft wax gourd against wilt diseases.

Methods

In the present study, disease and growth indices were compared between grafted and original wax gourds after infection with Fusarium. High level of disease resistance was observed in the grafted wax gourd, with a lower disease index and low impacts on growth after infection. RNA-seq was performed to identify the differentially expressed genes (DEGs) between the adjacent treatment time points in the grafted and original wax gourds, respectively. Then a comparative temporal analysis was performed and a total of 1,190 genes were identified to show different gene expression patterns between the two wax gourd groups during Fusarium infection.

Result and discussion

Here, high level of disease resistance was observed in the grafted wax gourd, with a lower disease index and low impacts on growth after infection. The DEG number was higher in grafted group than original group, and the enriched functional categories and pathways of DEGs were largely inconsistent between the two groups. These genes were enriched in multiple pathways, of which the mitogen-activated protein kinase (MAPK) signaling pathway enhanced the early defense response, and cutin, suberin, and wax biosynthesis signaling pathways enhanced surface resistance in grafted wax gourd in comparison to original group. Our study provides insights into the gene regulatory mechanisms underlying the resistance of grafted wax gourds to Fusarium wilt infection, which will facilitate the breeding and production of wilt-resistant rootstocks.

Pan-genome analysis sheds light on structural variation-based dissection of agronomic traits in melon crops

Sweetness and appearance of fresh fruits are key palatable and preference attributes for consumers and are often controlled by multiple genes. However, fine-mapping the key loci or genes of interest by single genome-based genetic analysis is challenging. Herein, we present the chromosome-level genome assembly of 1 landrace melon accession (Cucumis melo ssp. agrestis) with wild morphologic features and thus construct a melon pan-genome atlas via integrating sequenced melon genome datasets. Our comparative genomic analysis reveals a total of 3.4 million genetic variations, of which the presence/absence variations (PAVs) are mainly involved in regulating the function of genes for sucrose metabolism during melon domestication and improvement. We further resolved several loci that are accountable for sucrose contents, flesh color, rind stripe, and suture using a structural variation (SV)-based genome-wide association study. Furthermore, via bulked segregation analysis (BSA)-seq and map-based cloning, we uncovered that a single gene, (CmPIRL6), determines the edible or inedible characteristics of melon fruit exocarp. These findings provide important melon pan-genome information and provide a powerful toolkit for future pan-genome-informed cultivar breeding of melon.

Superflexible Artificial Soft Wood

Soft woods have attracted enormous interest due to their anisotropic cellular microstructure and unique flexibility. The conventional wood-like materials are usually subject to the conflict between the superflexibility and robustness. Inspired by the synergistic compositions of soft suberin and rigid lignin of cork wood which has good flexibility and mechanical robustness, an artificial soft wood is reported by freeze-casting the soft-in-rigid (rubber-in-resin) emulsions, where the carboxy nitrile rubber confers softness and rigid melamine resin provides stiffness. The subsequent thermal curing induces micro-scale phase inversion and leads to a continuous soft phase strengthened by interspersed rigid ingredients. The unique configuration ensures crack resistance, structural robustness and superb flexibility, including wide-angle bending, twisting, and stretching abilities in various directions, as well as excellent fatigue resistance and high strength, overwhelming the natural soft wood and most wood-inspired materials. This superflexible artificial soft wood represents a promising substrate for bending-insensitive stress sensors.

Nitrate_dependent suberization regulates cadmium uptake and accumulation in maize

In this study, effect of nitrate-dependent suberization in maize root on cadmium (Cd) uptake and accumulation was investigated. Suberization in maize roots was significantly lower in plants grown with a high nitrate supply compared with low nitrate. This decrease was seen in the total amount of suberin, in which the aliphatic suberin amount was significantly decreased, whereas no difference in aromatic suberin content between different N-treatments. RNA-sequencing showed that suberin biosynthesis genes were upregulated in low nitrate treatment, which correlated well with the increased suberin content. Bioimaging and xylem sap analysis showed that reduced exodermal and endodermal suberization in roots of plants grown under high nitrate promoted radial Cd transport along the crown root. The enhanced suberization in crown roots of plants grown in low nitrate restricted the radial transport of Cd from epidermis to cortex via decreased accessibility to Cd related transporters at the plasmalemma. Also, under low nitrate supply, the Cd transport gene ZmNramp5 was upregulated in the crown root, which may enhance Cd uptake by root tip where exodermis and endodermis were not fully suberized. These results suggest that high nitrate supply enhances Cd uptake and radial transport in maize roots by reducing exodermal and endodermal suberization.

Unusual late-fall wildfire in a pre-Alpine Fagus sylvatica forest reduced fine roots in the shallower soil layer and shifted very fine-root growth to deeper soil depth

After an unusual, late-fall wildfire in a European beech (Fagus sylvatica L.) forest in the pre-Alps of northern Italy, the finest roots (0‒0.3 mm diameter) were generally the most responsive to fire, with the effect more pronounced at the shallowest soil depth. While roots 0.3‒1 mm in diameter had their length and biomass at the shallowest soil depth reduced by fire, fire stimulated more length and biomass at the deepest soil depth compared to the control. Fire elevated the total length of dead roots and their biomass immediately and this result persisted through the first spring, after which control and fire-impacted trees had similar fine root turnover. Our results unveiled the fine-root response to fire when subdivided by diameter size and soil depth, adding to the paucity of data concerning fire impacts on beech roots in a natural condition and providing the basis for understanding unusual fire occurrence on root traits. This study suggests that F. sylvatica trees can adapt to wildfire by plastically changing the distribution of fine-root growth, indicating a resilience mechanism to disturbance.

Biocomposite Foams with Multimodal Cellular Structures Based on Cork Granulates and Microwave Processed Egg White Proteins

In an effort to reduce greenhouse gas emission, reduce the consumption of natural resources, and increase the sustainability of biocomposite foams, the present study focuses on the recycling of cork processing waste for the production of lightweight, non-structural, fireproof thermal and acoustic insulating panels. Egg white proteins (EWP) were used as a matrix model to introduce an open cell structure via a simple and energy-efficient microwave foaming process. Samples with different compositions (ratio of EWP and cork) and additives (eggshells and inorganic intumescent fillers) were prepared with the aim of correlating composition, cellular structures, flame resistance, and mechanical properties.

Characteristics of Biofoam Cups Made from Sugarcane Bagasse with Rhizopus oligosporus as Binding Agent

This study is aimed at producing a biofoam cup made from sugarcane bagasse with tempeh mold (Rhizopus oligosporus). Soybean flour (SF) was added to promote the growth of mycelia, which could bind the bagasse fiber matrix. The main materials were whole bagasse (B) and depithed bagasse (DB). The SF weight ratios to bagasse were 1 : 1 (SF1) and 1.5 : 1 (SF1.5). Therefore, the studied specimens were labeled B-SF1, DB-SF1, B-SF1.5, and DB-SF1.5. All biofoam cups were analyzed for their physical properties (water absorption and porosity), mechanical properties (puncture and compressive strengths), biodegradability, and thermal properties (thermogravimetric analysis). The lowest water absorption rates were obtained from the B biofoam cups ( $23\%\pm 2.45\%$ ) and the SF1.5 biofoam cups ( $25.83\%\pm 5.19\%$ ). Both B-SF1 and B-SF1.5 had lower porosity ( $8.72\%\pm 0.88\%$ and $10.77\%\pm 1.54\%$ , respectively) than the DB biofoam cups. Moreover, the B biofoam cups had smoother biofoam surfaces, smaller voids, and lower porosity compared with the DB samples. However, the DB biofoam cups showed the highest puncture strength ( $2.95\pm 0.37$  kg cm−2) among all samples. Nevertheless, the B-SF1.5 biofoam cup had the highest compressive strength ( $3.98\pm 0.39$  MPa) and the DB-SF1.5 exhibited the slowest degradation rate ( $27\%\pm 0.7\%$ ) after 14 days of soil burial. The highest thermal stability was obtained from B-SF1.5, which had a thermal degradation temperature of 264°C. Overall, B-SF1.5 had the smoothest surface, good thermal stability, and high compressive strength.

Characterization of MdMYB68, a suberin master regulator in russeted apples

INTRODUCTION

Apple russeting is mainly due to the accumulation of suberin in the cell wall in response to defects and damages in the cuticle layer. Over the last decades, massive efforts have been done to better understand the complex interplay between pathways involved in the suberization process in model plants. However, the regulation mechanisms which orchestrate this complex process are still under investigation. Our previous studies highlighted a number of transcription factor candidates from the Myeloblastosis (MYB) transcription factor family which might regulate suberization in russeted or suberized apple fruit skin. Among these, we identified MdMYB68, which was co-expressed with number of well-known key suberin biosynthesis genes.

METHOD

To validate the MdMYB68 function, we conducted an heterologous transient expression in Nicotiana benthamiana combined with whole gene expression profiling analysis (RNA-Seq), quantification of lipids and cell wall monosaccharides, and microscopy.

RESULTS

MdMYB68 overexpression is able to trigger the expression of the whole suberin biosynthesis pathway. The lipid content analysis confirmed that MdMYB68 regulates the deposition of suberin in cell walls. Furthermore, we also investigated the alteration of the non-lipid cell wall components and showed that MdMYB68 triggers a massive modification of hemicelluloses and pectins. These results were finally supported by the microscopy.

DISCUSSION

Once again, we demonstrated that the heterologous transient expression in N. benthamiana coupled with RNA-seq is a powerful and efficient tool to investigate the function of suberin related transcription factors. Here, we suggest MdMYB68 as a new regulator of the aliphatic and aromatic suberin deposition in apple fruit, and further describe, for the first time, rearrangements occurring in the carbohydrate cell wall matrix, preparing this suberin deposition.

Laser Microdissection of Woody and Suberized Plant Tissues for RNA-Seq Analysis

An accurate profile of gene expression at a cellular level can contribute to a better understanding of biological processes and complexities involved in regulatory mechanism of woody plants. Laser microdissection is one technique that allows isolation of specific, target cells or tissue from a heterogeneous cell population. This technique entails microscopic visualization of the selected tissue and use a laser beam to separate the desired cells from surrounding tissue. Initial identification of these cells is made based on morphology and/or histological staining. Some works have been made in several tissues and plant models. However, there are few studies of laser microdissection application in woody species, particularly, lignified and suberized cells. Moreover, the presence of high level of suberin in cell walls can be a big challenge for the application of this approach. In our study it was developed a technique for tissue isolation, using laser microdissection of four different plant cell types (phellogen, lenticels, cortex and xylem) from woody tissues of cork oak (Quercus suber), followed by RNA extraction and RNA-Seq. We tested several methodologies regarding laser microdissection, cryostat equipments, fixation treatments, duration of single-cells collection and number of isolated cells by laser microdissection and RNA extraction procedures. A simple and efficient protocol for tissue isolation by laser microdissection and RNA purification was obtained, with a final method validation of RNA-Seq analysis. The optimized methodology combining RNA-Seq for expression analysis will contribute to elucidate the molecular pathways associated with different development processes of the xylem and phellem in oaks, including the lenticular channels formation.

Cutinized and suberized barriers in leaves and roots: Similarities and differences

Anatomical, histochemical, chemical, and biosynthetic similarities and differences of cutinized and suberized plant cell walls are presented and reviewed in brief. Based on this, the functional properties of cutinized and suberized plant cell walls acting as transport barriers are compared and discussed in more detail. This is of general importance because fundamental misconceptions about relationships in plant-environment water relations are commonly encountered in the scientific literature. It will be shown here, that cuticles represent highly efficient apoplastic transport barriers significantly reducing the diffusion of water and dissolved compounds. The transport barrier of cuticles is mainly established by the deposition of cuticular waxes. Upon wax extraction, with the cutin polymer remaining, cuticular permeability for water and dissolved non-ionized and lipophilic solutes are increasing by 2-3 orders of magnitude, whereas polar and charged substances (e.g., nutrient ions) are only weakly affected (2- to 3-fold increases in permeability). Suberized apoplastic barriers without the deposition of wax are at least as permeable as the cutin polymer matrix without waxes and hardly offer any resistance to the free movement of water. Only upon the deposition of significant amounts of wax, as it is the case with suberized periderms exposed to the atmosphere, an efficient transport barrier for water can be established by suberized cell walls. Comparing the driving forces (gradients between water potentials inside leaves and roots and the surrounding environment) for water loss acting on leaves and roots, it is shown that leaves must have a genetically pre-defined highly efficient transpiration barrier fairly independent from rapidly changing environmental influences. Roots, in most conditions facing a soil environment with relative humidities very close to 100%, are orders of magnitude more permeable to water than leaf cuticles. Upon desiccation, the permanent wilting point of plants is defined as -1.5 MPa, which still corresponds to nearly 99% relative humidity in soil. Thus, the main reason for plant water stress leading to dehydration is the inability of root tissues to decrease their internal water potential to values more negative than -1.5 MPa and not the lack of a transport barrier for water in roots and leaves. Taken together, the commonly mentioned concepts that a drought-induced increase of cuticular wax or root suberin considerably strengthens the apoplastic leaf or root transport barriers and thus aids in water conservation appears highly questionable.

Differential Colonization of the Plant Vasculature Between Endophytic Versus Pathogenic Fusarium oxysporum Strains

Plant xylem colonization is the hallmark of vascular wilt diseases caused by phytopathogens within the Fusarium oxysporum species complex. Recently, xylem colonization has also been reported among endophytic F. oxysporum strains, resulting in some uncertainty. This study compares xylem colonization processes by pathogenic versus endophytic strains in Arabidopsis thaliana and Solanum lycopersicum, using Arabidopsis pathogen Fo5176, tomato pathogen Fol4287, and the endophyte Fo47, which can colonize both plant hosts. We observed that all strains were able to advance from epidermis to endodermis within 3 days postinoculation (dpi) and reached the root xylem at 4 dpi. However, this shared progression was restricted to lateral roots and the elongation zone of the primary root. Only pathogens reached the xylem above the primary-root maturation zone (PMZ). Related to the distinct colonization patterns, we also observed stronger induction of callose at the PMZ and lignin deposition at primary-lateral root junctions by the endophyte in both plants. This observation was further supported by stronger induction of Arabidopsis genes involved in callose and lignin biosynthesis during the endophytic colonization (Fo47) compared with the pathogenic interaction (Fo5176). Moreover, both pathogens encode more plant cell wall-degrading enzymes than the endophyte Fo47. Therefore, observed differences in callose and lignin deposition could be the combination of host production and the subsequent fungal degradation. In summary, this study demonstrates spatial differences between endophytic and pathogenic colonization, strongly suggesting that further investigations of molecular arm-races are needed to understand how plants differentiate friend from foe. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Plant root suberin: A layer of defence against biotic and abiotic stresses

Plant roots have important functions, such as acquiring nutrients and water from the surrounding soil and transporting them upwards to the shoots. Simultaneously, they must be able to exclude potentially harmful substances and prevent the entry of pathogens into the roots. The endodermis surrounds the vascular tissues and forms hydrophobic diffusion barriers including Casparian strips and suberin lamella. Suberin in cell walls can be induced by a range of environmental factors and contribute to against biotic and abiotic threats. Tremendous progress has been made in biosynthesis of suberin and its function, little is known about the effect of its plasticity and distribution on stress tolerance. In field conditions, biotic and abiotic stress can exist at the same time, and little is known about the change of suberization under that condition. This paper update the progress of research related to suberin biosynthesis and its function, and also discuss the change of suberization in plant roots and its role on biotic and abiotic stresses tolerance.

The Streptomyces scabiei Pathogenicity Factor Thaxtomin A Induces the Production of Phenolic Compounds in Potato Tubers

The phytotoxin thaxtomin A (TA) is the key pathogenicity factor synthesized by the bacteria Streptomyces scabiei, the main causal agent of common scab of potato (Solanum tuberosum L.). TA treatment of potato tuber flesh produces a brown color that was attributed to necrosis. The intensity of TA-induced browning was generally thought to correlate with potato sensitivity to the disease. In this study, we found that TA-induced browning was much more intense in the potato tuber flesh of the common scab moderately resistant variety Russet Burbank (RB) than that observed in tubers of the disease-susceptible variety Yukon Gold (YG). However, there was no significant difference in the level of TA-induced cell death detected in both varieties, suggesting that tubers response to TA does not correlate with the level of sensitivity to common scab. TA-treated potato tuber tissues accumulated significantly higher levels of phenolic compounds than untreated controls, with a higher phenol content detected in RB TA-treated tissues than in those of YG. Browning was associated with a significant induction of the expression of genes of the phenylpropanoid pathway in RB tubers, indicating that TA activated this metabolic pathway. These results suggest that tuber flesh browning induced by TA is due to the accumulation of phenolic compounds. These phenolics may play a role in the protection of potato tubers against S. scabiei.

Beyond width and density: stable carbon and oxygen isotopes in cork-rings provide insights of physiological responses to water stress in Quercus suber L

As climate change increasingly affects forest ecosystems, detailed understanding of major effects is important to anticipate their consequences under future climate scenarios. The Mediterranean region is a prominent climate change hotspot, and evergreen cork oak (Quercus suber L.) woodlands are particularly climatically sensitive due to cork (bark) harvesting. Cork oak's drought avoidance strategy is well-known and includes structural and physiological adaptations that maximise soil water uptake and transport and limit water use, potentially leading to reduced stem and cork growth. Trees' responses to cope with water-limited conditions have been extensively described based on cork-rings width and, more recently, on cork-rings density, in dendroecological studies. However, so far, tree functional attributes and physiological strategies, namely photosynthetic metabolism adjustments affecting cork formation, have never been addressed and/or integrated on these previous cork-rings-based studies. In this study, we address the relation between carbon and oxygen stable isotopes of cork rings and precipitation and temperature, in two distinct locations of southwestern Portugal-the (wetter) Tagus basin peneplain and the (drier) Grândola mountains. We aimed at assessing whether the two climatic factors affect cork-ring isotopic composition under contrasting conditions of water availability, and, therefore, if carbon and oxygen signatures in cork can reflect tree functional (physiological and structural) responses to stressful conditions, which might be aggravated by climate change. Our results indicate differences between the study areas. At the drier site, the stronger statistically significant negative cork δ ¹³C correlations were found with mean temperature, whereas strong positive cork δ ¹⁸O correlations were fewer and found only with precipitation. Moreover, at the wetter site, cork rings are enriched in ¹⁸O and depleted in ¹³C, indicating, respectively, shallow groundwater as the water source for physiological processes related with biosynthesis of non-photosynthetic secondary tissues, such as suberin, and a weak stomatal regulation under high water availability, consistent with non-existent water availability constrains. In contrast, at the drier site, trees use water from deeper ground layers, depleted in ¹⁸O, and strongly regulate stomatal conductance under water stress, thus reducing photosynthetic carbon uptake and probably relying on stored carbon reserves for cork ring formation. These results suggest that although stable isotopes signatures in cork rings are not proxies for net growth, they may be (fairly) robust indicators of trees' physiological and structural adjustments to climate and environmental changes in Mediterranean environments.

MdMYB52 regulates lignin biosynthesis upon the suberization process in apple

Our previous studies, comparing russeted vs. waxy apple skin, highlighted a MYeloBlastosys (Myb) transcription factor (MdMYB52), which displayed a correlation with genes associated to the suberization process. The present article aims to assess its role and function in the suberization process. Phylogenetic analyses and research against Arabidopsis thaliana MYBs database were first performed and the tissue specific expression of MdMYB52 was investigated using RT-qPCR. The function of MdMYB52 was further investigated using Agrobacterium-mediated transient overexpression in Nicotiana benthamiana leaves. An RNA-Seq analysis was performed to highlight differentially regulated genes in response MdMYB52. Transcriptomic data were supported by analytical chemistry and microscopy. A massive decreased expression of photosynthetic and primary metabolism pathways was observed with a concomitant increased expression of genes associated with phenylpropanoid and lignin biosynthesis, cell wall modification and senescence. Interestingly key genes involved in the synthesis of suberin phenolic components were observed. The analytical chemistry displayed a strong increase in the lignin content in the cell walls during MdMYB52 expression. More specifically, an enrichment in G-Unit lignin residues was observed, supporting transcriptomic data as well as previous work describing the suberin phenolic domain as a G-unit enriched lignin-like polymer. The time-course qPCR analysis revealed that the observed stress response, might be explain by this lignin biosynthesis and by a possible programmed senescence triggered by MdMYB52. The present work supports a crucial regulatory role for MdMYB52 in the biosynthesis of the suberin phenolic domain and possibly in the fate of suberized cells in russeted apple skins.

Solving the regulation puzzle of periderm development using advances in fruit skin

Periderm protects enlarged organs of most dicots and gymnosperms as a barrier to water loss and disease invasion during their secondary growth. Its development undergoes a complex process with genetically controlled and environmental stress-induced characters. Different development of periderm makes the full and partial russet of fruit skin, which diverges in inheritance with qualitative and quantitative characters, respectively, in pear pome. In addition to its specific genetics, fruit periderm has similar development and structure as that of stem and other organs, making it an appropriate material for periderm research. Recently, progress in histochemical as well as transcriptome and proteome analyses, and quantitative trait locus (QTL) mapping have revealed the regulatory molecular mechanism in the periderm based on the identification of switch genes. In this review, we concentrate on the periderm development, propose the conservation of periderm regulation between fruit and other plant organs based on their morphological and molecular characteristics, and summarize a regulatory network with the elicitors and repressors for the tissue development. Spontaneous programmed-cell death (PCD) or environmental stress produces the original signal that triggers the development of periderm. Spatio-temporal specific PCD produced by PyPPCD1 gene and its homologs can play a key role in the coordinated regulation of cell death related tissue development.

Potato Periderm Development and Tuber Skin Quality

The periderm is a corky tissue that replaces the epidermis when the latter is damaged, and is critical for preventing pathogen invasion and water loss. The periderm is formed through the meristematic activity of phellogen cells (cork cambium). The potato skin (phellem cells) composes the outer layers of the tuber periderm and is a model for studying cork development. Early in tuber development and following tuber expansion, the phellogen becomes active and produces the skin. New skin layers are continuously added by division of the phellogen cells until tuber maturation. Some physiological disorders of the potato tuber are related to abnormal development of the skin, including skinning injuries and russeting of smooth-skinned potatoes. Thus, characterizing the potato periderm contributes to modeling cork development in plants and helps to resolve critical agricultural problems. Here, we summarize the data available on potato periderm formation, highlighting tissue characteristics rather than the suberization processes.

The making of suberin

Outer protective barriers of animals use a variety of bio-polymers, based on either proteins (e.g. collagens), or modified sugars (e.g. chitin). Plants, however, have come up with a particular solution, based on the polymerisation of lipid-like precursors, giving rise to cutin and suberin. Suberin is a structural lipophilic polyester of fatty acids, glycerol and some aromatics found in cell walls of phellem, endodermis, exodermis, wound tissues, abscission zones, bundle sheath and other tissues. It deposits as a hydrophobic layer between the (ligno)cellulosic primary cell wall and plasma membrane. Suberin is highly protective against biotic and abiotic stresses, shows great developmental plasticity and its chemically recalcitrant nature might assist the sequestration of atmospheric carbon by plants. The aim of this review is to integrate the rapidly accelerating genetic and cell biological discoveries of recent years with the important chemical and structural contributions obtained from very diverse organisms and tissue layers. We critically discuss the order and localisation of the enzymatic machinery synthesising the presumed substrates for export and apoplastic polymerisation. We attempt to explain observed suberin linkages by diverse enzyme activities and discuss the spatiotemporal relationship of suberin with lignin and ferulates, necessary to produce a functional suberised cell wall.

Comparative transcriptomics of tropical woody plants supports fast and furious strategy along the leaf economics spectrum in lianas

Lianas, climbing woody plants, influence the structure and function of tropical forests. Climbing traits have evolved multiple times, including ancestral groups such as gymnosperms and pteridophytes, but the genetic basis of the liana strategy is largely unknown. Here, we use a comparative transcriptomic approach for 47 tropical plant species, including ten lianas of diverse taxonomic origins, to identify genes that are consistently expressed or downregulated only in lianas. Our comparative analysis of full-length transcripts enabled the identification of a core interactomic network common to lianas. Sets of transcripts identified from our analysis reveal features related to functional traits pertinent to leaf economics spectrum in lianas, include upregulation of genes controlling epidermal cuticular properties, cell wall remodeling, carbon concentrating mechanism, cell cycle progression, DNA repair and a large suit of downregulated transcription factors and enzymes involved in ABA-mediated stress response as well as lignin and suberin synthesis. All together, these genes are known to be significant in shaping plant morphologies through responses such as gravitropism, phyllotaxy and shade avoidance.

Summary: The full-length fraction of liana transcriptomes mapped on a protein–protein interactome revealed the nature of their convergence through distinct sets of expressed and downregulated genes not observed in free-standing plants.

Utilization of Suberinic Acids Containing Residue as an Adhesive for Particle Boards

The birch (Betula spp.) outer bark is a valuable product rich in betulin. After removal of betulin extractives, suberin containing tissues are left. Suberin is a biopolyester built from α,ω-bifunctional fatty acids (suberinic acids), which after depolymerization together with lignocarbohydrate complex is a potential adhesive as a side-stream product (residue) from obtaining suberinic acids for polyol synthesis. In this work, we studied the utilization possibilities in particleboards of the said residue obtained by depolymerization in four different solvents (methanol, ethanol, isopropanol and 1-butanol). The adhesives were characterised by chemical (acid number, solubility in tetrahydrofuran, epoxy and ash content) and instrumental analytical methods (SEC-RID, DSC, TGA and FTIR). Based on the results of mechanical characteristics, ethanol was chosen as the most suitable depolymerization medium. The optimal hot-pressing parameters for particleboards were determined using the design of experiments approach: adhesive content 20 wt%; hot-pressing temperature 248 °C, and hot-pressing time 6.55 min.

The Making of Plant Armor: The Periderm

The periderm acts as armor protecting the plant's inner tissues from biotic and abiotic stress. It forms during the radial thickening of plant organs such as stems and roots and replaces the function of primary protective tissues such as the epidermis and the endodermis. A wound periderm also forms to heal and protect injured tissues. The periderm comprises a meristematic tissue called the phellogen, or cork cambium, and its derivatives: the lignosuberized phellem and the phelloderm. Research on the periderm has mainly focused on the chemical composition of the phellem due to its relevance as a raw material for industrial processes. Today, there is increasing interest in the regulatory network underlying periderm development as a novel breeding trait to improve plant resilience and to sequester CO₂. Here, we discuss our current understanding of periderm formation, focusing on aspects of periderm evolution, mechanisms of periderm ontogenesis, regulatory networks underlying phellogen initiation and cork differentiation, and future challenges of periderm research.

Identification of genes and metabolic pathways involved in wounding-induced kiwifruit ripening

Fruit is constantly challenged by wounding events, inducing accelerated ripening and irreversible metabolic changes. However, cognate mechanisms that regulate this process are little known. To expand our knowledge of ripening metabolism induced by wounding, an artificial-wound global transcriptome investigation combined with metabolite profiling study was conducted in postharvest kiwifruit (Actinidia chinensis var. deliciosa (A. Chev.) A. Chev. 'Hayward'). Wounding treatment promoted fruit ripening, as demonstrated by changes in fruit firmness, ethylene production and respiration activity determined periodically during a ripening period of 8 d at room temperature. Calcium imaging using fluorescent probe Fluo-3 AM revealed spatial dynamics of Ca²⁺ signaling in the wounding area following 8d ripening. Several sugars including fructose, glucose, and sucrose as well as organic acids such as citric, succinic and galacturonic acid were increased by wounding. Changes of various amino acids in wounded-treated fruit, especially 5-oxoproline and valine along with alternations of soluble alcohols, like myo-inositol were detected. Gene expression analysis of the wounded fruit showed increased expression of genes that are mainly involved in defense response (e.g., AdTLP.1-3, AdPP2C.1-2, AdMALD1), calcium ion binding (e.g., AdCbEFh, AdCLR, AdANX), TCA cycle (e.g., AdMDH.1, AdMDH.2, AdCS), sugars (e.g., AdSUSA.1, AdSPS4, AdABFr), secondary metabolism (e.g., AdPAL.1-3, AdCCR, AdHCT.1-2), lipid processing (e.g., AdGELP.1-4, AdGELP) and pectin degradation (e.g., AdPE.1-2, AdPAE.1-2, AdPG.1-2) as well as in ethylene (AdERF7, AdERF1B, AdACO.1-4) and auxin (AdICE, AdAEFc, AdASII) synthesis and perception. Moreover, genes related to aquaporins, such as AdAQP2, AdAQP4 and AdAQP7 were down-regulated in fruit exposed to wounding. These results demonstrate multiple metabolic points of wounding regulatory control during kiwifruit ripening and provide insights into the molecular basis of wounding-mediated ripening.

Biomass Pre-Extraction as a Versatile Strategy to Improve Biorefinery Feedstock Flexibility, Sugar Yields, and Lignin Purity

Feedstock flexibility is highly advantageous for the viability of (solvent-based) biorefineries but comes with the considerable challenge of having to cope with the varying nature and typically high abundance of nonlignocellulose compounds in the most readily available residual biomass streams. Here, we demonstrate that mild aqueous acetone organosolv fractionation of various complex lignocellulosic raw materials (roadside grass, wheat straw, birch branches, almond shells, and a mixed stream thereof) is indeed negatively affected by these compounds and present a versatile strategy to mitigate this bottleneck in biorefining. A biomass pre-extraction approach has been developed to remove the detrimental extractives with (aqueous) acetone prior to fractionation. Pre-extraction removed organic extractives as well as minerals, primarily reducing acid dose requirements for fractionation and loss of hemicellulose sugars by degradation and improved the purity of the isolated lignin. We show how pre-extraction affects the effectiveness of the biorefinery process, including detailed mass balances for pretreatment, downstream processing, and product characteristics, and how it affects solvent and energy use with a first conceptual process design. The integrated biorefining approach allows for the improved compatibility of biorefineries with sustainable feedstock supply chains, enhanced biomass valorization (i.e., isolation of bioactive compounds from the extract), and more effective biomass processing with limited variation in product quality.

Quercus suber: A Promising Sustainable Raw Material for Cosmetic Application

There is a drive within the cosmetic industry towards the development of more sustainable products, supported by consumer awareness of the environmental footprint. The cosmetic industry is rising to meet consumer demand by following practices, such as the use of by-products from agro-industrial waste. Quercus suber is a tree prevalent in the Mediterranean basin. The extraction of cork is considered sustainable, as this process does not harm the tree, and the amount of cork produced increases with the number of extractions. Beyond this, the cork industry produces by-products that are used to sustain the industry itself, such as cork powder, which is reused for generating energy. Additionally, cork and cork by-products contain bioactive compounds mainly with antioxidant activity that can be of use to the cosmetic industry, such as for antiaging, anti-acne, anti-inflammatory, and depigmenting cosmetic products. We provide the reader with an overview of the putative cosmetic applications of cork and its by-products as well as of their bioactive compounds. It is noteworthy that only a few cork-based cosmetic products have reached the market, namely antiaging and exfoliant products. Clearly, the use of cork upcycled cosmetic ingredients will evolve in the future considering the wide array of biological activities already reported.

Induced ligno-suberin vascular coating and tyramine-derived hydroxycinnamic acid amides restrict Ralstonia solanacearum colonization in resistant tomato

Tomato varieties resistant to the bacterial wilt pathogen Ralstonia solanacearum have the ability to restrict bacterial movement in the plant. Inducible vascular cell wall reinforcements seem to play a key role in confining R. solanacearum into the xylem vasculature of resistant tomato. However, the type of compounds involved in such vascular physico-chemical barriers remain understudied, while being a key component of resistance. Here we use a combination of histological and live-imaging techniques, together with spectroscopy and gene expression analysis to understand the nature of R. solanacearum-induced formation of vascular coatings in resistant tomato. We describe that resistant tomato specifically responds to infection by assembling a vascular structural barrier formed by a ligno-suberin coating and tyramine-derived hydroxycinnamic acid amides. Further, we show that overexpressing genes of the ligno-suberin pathway in a commercial susceptible variety of tomato restricts R. solanacearum movement inside the plant and slows disease progression, enhancing resistance to the pathogen. We propose that the induced barrier in resistant plants does not only restrict the movement of the pathogen, but may also prevent cell wall degradation by the pathogen and confer anti-microbial properties, effectively contributing to resistance.

Translational profile of developing phellem cells in Arabidopsis thaliana roots

The phellem is a specialized boundary tissue providing the first line of defense against abiotic and biotic stresses in organs undergoing secondary growth. Phellem cells undergo several differentiation steps, which include cell wall suberization, cell expansion, and programmed cell death. Yet, the molecular players acting particularly in phellem cell differentiation remain poorly described, particularly in the widely used model plant Arabidopsis thaliana. Using specific marker lines we followed the onset and progression of phellem differentiation in A. thaliana roots and further targeted the translatome of newly developed phellem cells using translating ribosome affinity purification followed by mRNA sequencing (TRAP-SEQ). We showed that phellem suberization is initiated early after phellogen (cork cambium) division. The specific translational landscape was organized in three main domains related to energy production, synthesis and transport of cell wall components, and response to stimulus. Novel players in phellem differentiation related to suberin monomer transport and assembly as well as novel transcription regulators were identified. This strategy provided an unprecedented resolution of the translatome of developing phellem cells, giving a detailed and specific view on the molecular mechanisms acting on cell differentiation in periderm tissues of the model plant Arabidopsis.

Impairment of the cellulose degradation machinery enhances Fusarium oxysporum virulence but limits its reproductive fitness

Fungal pathogens grow in the apoplastic space, in constant contact with the plant cell wall (CW) that hinders microbe progression while representing a source of nutrients. Although numerous fungal CW modifying proteins have been identified, their role during host colonization remains underexplored. Here, we show that the root-infecting plant pathogen Fusarium oxysporum (Fo) does not require its complete arsenal of cellulases to infect the host plant. Quite the opposite: Fo mutants impaired in cellulose degradation become hypervirulent by enhancing the secretion of virulence factors. On the other hand, the reduction in cellulase activity had a severe negative effect on saprophytic growth and microconidia production during the final stages of the Fo infection cycle. These findings enhance our understanding of the function of plant CW degradation on the outcome of host-microbe interactions and reveal an unexpected role of cellulose degradation in a pathogen's reproductive success.

Chemical and Molecular Characterization of Wound-Induced Suberization in Poplar (Populus alba × P. tremula) Stem Bark

Upon mechanical damage, plants produce wound responses to protect internal tissues from infections and desiccation. Suberin, a heteropolymer found on the inner face of primary cell walls, is deposited in specific tissues under normal development, enhanced under abiotic stress conditions and synthesized by any tissue upon mechanical damage. Wound-healing suberization of tree bark has been investigated at the anatomical level but very little is known about the molecular mechanisms underlying this important stress response. Here, we investigated a time course of wound-induced suberization in poplar bark. Microscopic changes showed that polyphenolics accumulate 3 days post wounding, with aliphatic suberin deposition observed 5 days post wounding. A wound periderm was formed 9 days post wounding. Chemical analyses of the suberin polyester accumulated during the wound-healing response indicated that suberin monomers increased from 0.25 to 7.98 mg/g DW for days 0 to 28, respectively. Monomer proportions varied across the wound-healing process, with an overall ratio of 2:1 (monomers:glycerol) found across the first 14 days post wounding, with this ratio increasing to 7:2 by day 28. The expression of selected candidate genes of poplar suberin metabolism was investigated using qRT-PCR. Genes queried belonging to lipid polyester and phenylpropanoid metabolism appeared to have redundant functions in native and wound-induced suberization. Our data show that, anatomically, the wounding response in poplar bark is similar to that described in periderms of other species. It also provides novel insight into this process at the chemical and molecular levels, which have not been previously studied in trees.

Suberin Biosynthesis, Assembly, and Regulation

Suberin is a specialized cell wall modifying polymer comprising both phenolic-derived and fatty acid-derived monomers, which is deposited in below-ground dermal tissues (epidermis, endodermis, periderm) and above-ground periderm (i.e., bark). Suberized cells are largely impermeable to water and provide a critical protective layer preventing water loss and pathogen infection. The deposition of suberin is part of the skin maturation process of important tuber crops such as potato and can affect storage longevity. Historically, the term "suberin" has been used to describe a polyester of largely aliphatic monomers (fatty acids, ω-hydroxy fatty acids, α,ω-dioic acids, 1-alkanols), hydroxycinnamic acids, and glycerol. However, exhaustive alkaline hydrolysis, which removes esterified aliphatics and phenolics from suberized tissue, reveals a core poly(phenolic) macromolecule, the depolymerization of which yields phenolics not found in the aliphatic polyester. Time course analysis of suberin deposition, at both the transcriptional and metabolite levels, supports a temporal regulation of suberin deposition, with phenolics being polymerized into a poly(phenolic) domain in advance of the bulk of the poly(aliphatics) that characterize suberized cells. In the present review, we summarize the literature describing suberin monomer biosynthesis and speculate on aspects of suberin assembly. In addition, we highlight recent advances in our understanding of how suberization may be regulated, including at the phytohormone, transcription factor, and protein scaffold levels.

Building Blocks of the Protective Suberin Plant Polymer Self-Assemble into Lamellar Structures with Antibacterial Potential

The protection of terrestrial plants from desiccation, mechanical injury, and pathogenic invasion is achieved by waxes and cutin polyesters on leaf and fruit surfaces as well as suberin polymers that are embedded in the cell walls of roots, but the physicochemical principles governing the organization of these biological composites remain incompletely understood. Despite the well-established enzymatic mediation of suberin formation in the skins of potato tubers, cork oak trees, and internal plant tissues, the additional possibility of self-assembly in this system was suggested by our serendipitous finding that solvent extracts from potato phellem tissues form suspended fibers and needles in the absence of such catalysts over a period of several weeks. In the current study, we investigated self-assembly for three-component model chemical mixtures comprised of a hydroxyfatty acid, glycerol, and either of two hydroxycinnamic acids that together typify the building blocks of potato suberin biopolymers. We demonstrate that these mixtures spontaneously form lamellar structures that are reminiscent of suberized plant tissues, incorporate all constituents into self-assemblies, can form covalently bound ester structures, and display antibacterial activity. These findings provide new perspectives on the self-association and reactivity of these classes of organic compounds, insights into agriculturally important suberin formation in food crops, and a starting point for engineering sustainable materials with antimicrobial capabilities.

Water stress resilient cereal crops: Lessons from wild relatives

Cereal crops are significant contributors to global diets. As climate change disrupts weather patterns and wreaks havoc on crops, the need for generating stress-resilient, high-yielding varieties is more urgent than ever. One extremely promising avenue in this regard is to exploit the tremendous genetic diversity expressed by the wild ancestors of current day crop species. These crop wild relatives thrive in a range of environments and accordingly often harbor an array of traits that allow them to do so. The identification and introgression of these traits into our staple cereal crops can lessen yield losses in stressful environments. In the last decades, a surge in extreme drought and flooding events have severely impacted cereal crop production. Climate models predict a persistence of this trend, thus reinforcing the need for research on water stress resilience. Here we review: (i) how water stress (drought and flooding) impacts crop performance; and (ii) how identification of tolerance traits and mechanisms from wild relatives of the main cereal crops, that is, rice, maize, wheat, and barley, can lead to improved survival and sustained yields in these crops under water stress conditions.

The Key Enzymes in the Suberin Biosynthetic Pathway in Plants: An Update

Suberin is a natural biopolymer found in a variety of specialized tissues, including seed coat integuments, root endodermis, tree bark, potato tuber skin and the russeted and reticulated skin of fruits. The suberin polymer consists of polyaliphatic and polyphenolic domains. The former is made of very long chain fatty acids, primary alcohols and a glycerol backbone, while the latter consists of p-hydroxycinnamic acid derivatives, which originate from the core phenylpropanoid pathway. In the current review, we survey the current knowledge on genes/enzymes associated with the suberin biosynthetic pathway in plants, reflecting the outcomes of considerable research efforts in the last two decades. We discuss the function of these genes/enzymes with respect to suberin aromatic and aliphatic monomer biosynthesis, suberin monomer transport, and suberin pathway regulation. We also delineate the consequences of the altered expression/accumulation of these genes/enzymes in transgenic plants.

Genetic and molecular mechanisms underlying mangrove adaptations to intertidal environments

Mangroves are halophytic plants belonging to diverse angiosperm families that are adapted to highly stressful intertidal zones between land and sea. They are special, unique, and one of the most productive ecosystems that play enormous ecological roles and provide a large number of benefits to the coastal communities. To thrive under highly stressful conditions, mangroves have innovated several key morphological, anatomical, and physio-biochemical adaptations. The evolution of the unique adaptive modifications might have resulted from a host of genetic and molecular changes and to date we know little about the nature of these genetic and molecular changes. Although slow, new information has accumulated over the last few decades on the genetic and molecular regulation of the mangrove adaptations, a comprehensive review on it is not yet available. This review provides up-to-date consolidated information on the genetic, epigenetic, and molecular regulation of mangrove adaptive traits.

Hydroponic cultivation conditions allowing the reproducible investigation of poplar root suberization and water transport

BACKGROUND

With increasing joint research cooperation on national and international levels, there is a high need for harmonized and reproducible cultivation conditions and experimental protocols in order to ensure the best comparability and reliability of acquired data. As a result, not only comparisons of findings of different laboratories working with the same species but also of entirely different species would be facilitated. As Populus is becoming an increasingly important genus in modern science and agroforestry, the integration of findings with previously gained knowledge of other crop species is of high significance.

RESULTS

To ease and ensure the comparability of investigations of root suberization and water transport, on a high degree of methodological reproducibility, we set up a hydroponics-based experimental pipeline. This includes plant cultivation, root histochemistry, analytical investigation, and root water transport measurement. A 5-week-long hydroponic cultivation period including an optional final week of stress application resulted in a highly consistent poplar root development. The poplar roots were of conical geometry and exhibited a typical Casparian band development with subsequent continuously increasing suberization of the endodermis. Poplar root suberin was composed of the most frequently described suberin substance classes, but also high amounts of benzoic acid derivatives could be identified. Root transport physiology experiments revealed that poplar roots in this developmental stage have a two- to tenfold higher hydrostatic than osmotic hydraulic conductivity. Lastly, the hydroponic cultivation allowed the application of gradually defined osmotic stress conditions illustrating the precise adjustability of hydroponic experiments as well as the previously reported sensitivity of poplar plants to water deficits.

CONCLUSIONS

By maintaining a high degree of harmonization, we were able to compare our results to previously published data on root suberization and water transport of barley and other crop species. Regarding hydroponic poplar cultivation, we enabled high reliability, reproducibility, and comparability for future experiments. In contrast to abiotic stress conditions applied during axenic tissue culture cultivation, this experimental pipeline offers great advantages including the growth of roots in the dark, easy access to root systems before, during, and after stress conditions, and the more accurate definition of the developmental stages of the roots.

Dynamics of Leaf- and Root-Specific Biomarkers during 1-Year of Litter Decomposition

Root-specific and leaf-specific biomarkers have been used for decades to identify the origin of organic materials in soils and sediments. However, quantitative approaches require appropriate knowledge about the fate of these indicator molecules during degradation. To clarify this issue, we performed a 1-year incubation experiment with fine root and leaf material of six temperate tree species: European ash (Fraxinus excelsior), European beech (Fagus sylvatica), Oak spec. (Quercus spec.), Linden spec. (Tilia spec.), Norway spruce (Picea abies) and Scots pine (Pinus sylvatica). Only one molecule, x,16-dihydroxy hexadecanoic acid (x,16-C16), could be validated as a general leaf-specific biomarker for the set of all species. For roots, no general root biomarker was found. Ester-bound tricosanol (C23-OH) could be validated for five out of six species; 20-hydroxy eicosanoic acid (ωC20) could be validated for four out of six species, leaving Norway spruce without a suitable root biomarker. The results of this study suggest that the validity of leaf- and root-derived ester-bound lipids as biomarkers is highly species dependent and does not always coincide with previous findings. Concentrations of root- and leaf-derived ester-bound lipids did not stay constant within 1 year of degradation and changed without a linear trend. The change of concentrations seems to be highly species dependent. This might be due to a different structure and arrangement of the individual monomers in cutin and suberin per species, and, therefore, a different accessibility of bond cleaving enzymes. The usefulness of root and leaf biomarkers is context dependent. Our results suggest that general assumptions about litter input to forest soils solely based on biomarker analysis have to be considered carefully.

Multistage fractionation of pine bark by liquid and supercritical carbon dioxide

Multistage fractionation of pine bark was performed using subcritical and supercritical CO₂ at increasing pressures and temperatures. In total, seven fractions were collected, which demonstrated different enrichments of families of compounds. In particular, subcritical CO₂ yielded 41% of the total extract in which unsaturated fatty acids represented the most abundant family. The subsequent five supercritical steps increased the recovery of sterol esters, wax esters and resin acids at higher temperatures and pressures, reaching 80% of the total extractable mass. In the last step, using ethanol as a co-solvent, an additional 20% of extract was recovered, which was enriched with phenolics and glycerol. A full characterisation of the extracts was accomplished by high-temperature GC-MS/FID using four internal standards, which were representative of the main classes of compounds contained in the pine bark extract.

Root Suberin Plays Important Roles in Reducing Water Loss and Sodium Uptake in Arabidopsis thaliana

Suberin is a cell-wall-associated hetero-polymer deposited in specific plant tissues. The precise role of its composition and lamellae structure in protecting plants against abiotic stresses is unclear. In Arabidopsis thaliana, we tested the biochemical and physiological responses to water deficiency and NaCl treatment in mutants that are differentially affected in suberin composition and lamellae structure. Chronic drought stress increased suberin and suberin-associated waxes in wild-type plants. Suberin-deficient mutants were not more susceptible than the wild-type to the chronic drought stress imposed in this study. Nonetheless, the cyp86a1-1 cyp86b1-1 mutant, which had a severely altered suberin composition and lamellae structure, exhibited increased water loss through the root periderm. Cyp86a1-1 cyp86b1-1 also recorded lower relative water content in leaves. The abcg2-1 abcg6-1 abcg20-1 mutant, which has altered suberin composition and lamellae, was very sensitive to NaCl treatment. Furthermore, cyp86a1-1 cyp86b1-1 recorded a significant drop in the leaf K/Na ratio, indicating salt sensitivity. The far1-2 far4-1 far5-1 mutant, which did not show structural defects in the suberin lamellae, had similar responses to drought and NaCl treatments as the wild-type. Our results provide evidence that the suberin amount and lamellae structure are key features in the barrier function of suberin in reducing water loss and reducing sodium uptake through roots for better performance under drought and salt stresses.

Borate and phosphite treatments of potato plants (Solanum tuberosum L.) as a proof of concept to reinforce the cell wall structure and reduce starch digestibility

Potatoes are one of the main sources of carbohydrates in human diet, however they have a high glycaemic index (GI). Hence, developing new agricultural and industrial strategies to produce low GI potatoes represents a health priority to prevent obesity and related diseases. In this work, we investigated whether treatments of potato plants with elicitors of plant defence responses can lead to a reduction of tuber starch availability and digestibility, through the induction of cell wall remodelling and stiffening. Treatments with phosphites (KPhi) and borate were performed, as they are known to activate plant defence responses that cause modifications in the architecture and composition of the plant cell wall. Data of suberin autofluorescence demonstrated that potato plants grown in a nutrition medium supplemented with KPhi and borate produced tubers with a thicker periderm, while pectin staining demonstrated that KPhi treatment induced a reinforcement of the wall of storage parenchyma cells. Both compounds elicited the production of H₂O₂, which is usually involved in cell-wall remodelling and stiffening reactions while only KPhi caused an increase of the total content of phenolic compounds. A two-phase digestion in vitro assay showed that treatment with KPhi determined a significant decrease of the starch hydrolysis rate in potato tubers. This work highlights the ability of cell wall architecture in modulating starch accessibility to digestive enzymes, paving the way for new agronomic practices to produce low GI index potatoes.

A chemical window into the impact of RNAi silencing of the StNAC103 gene in potato tuber periderms: Soluble metabolites, suberized cell walls, and antibacterial defense

The growth and survival of terrestrial plants require control of their interactions with the environment, e.g., to defend against desiccation and microbial invasion. For major food crops, the protection conferred by the outer skins (periderm in potato) is essential to cultivation, storage, and marketing of the edible tubers and fruits. Potatoes are particularly vulnerable to bacterial infections due to their high content of water and susceptibility to mechanical wounding. Recently, both specific and conserved gene silencing (StNAC103-RNAi and StNAC103-RNAi-c, respectively) were found to increase the load of wax and aliphatic suberin depolymerization products in tuber periderm, implicating this NAC gene as a repressor of the wax and suberin biosynthetic pathways. However, an important gap in our understanding of StNAC103 silencing concerns the metabolites produced in periderm cells as antimicrobial defense agents and potential building blocks of the deposited suberin biopolymer. In the current work, we have expanded prior studies on StNAC103 silenced lines by conducting comprehensive parallel analyses to profile changes in chemical constituents and antibacterial activity. Compositional analysis of the intact suberized cell walls using solid-state ¹³C NMR (ssNMR) showed that NAC silencing produced an increase in the long-chain aliphatic groups deposited within the periderm cell walls. LC-MS of polar extracts revealed up-regulation of glycoalkaloids in both StNAC103-RNAi and StNAC103-RNAi-c native periderms but down-regulation of a phenolic amine in StNAC103-RNAi-c and a phenolic acid in StNAC103-RNAi native periderms. The nonpolar soluble metabolites identified using GC-MS included notably abundant long-chain alkane metabolites in both silenced samples. By coordinating the differentially accumulated soluble metabolites and the suberin depolymerization products with the ssNMR-based profiles for the periderm polymers, it was possible to obtain a holistic view of the chemical changes that result from StNAC103 gene silencing. Correspondingly, the chemical composition trends served as a backdrop to interpret trends in the chemical barrier defense function of native tuber periderms, which was found to be more robust for the nonpolar extracts.

Structural Elucidation of Suberin from the Bark of Cultivated Willow (Salix sp.)

Although extractives have been symbolized as major bioactive pharmacological compounds from Salix (Salicaceae) bark, we speculated that these pharmaceutical effects cannot be solely attributed to phenolic components and their derivatives, but the long-chain suberin acids also contribute to their therapeutic effects. Hence, isolation and deconstruction of suberin were conducted, for the first time, to enrich our knowledge about the macromolecular components at the cell wall of willow bark. Saponification was adopted to obtain suberin extracts at a yield of approximately 5 wt % based on the bark of the studied hybrids. Gas chromatography-mass spectrometry allowed qualification and quantification of 23 compounds from the released suberin monomers, from which fatty acids represented majority of the isolated suberin, namely, fatty acid methyl esters (C17-C19); mono-carboxylic acid (C7-C16); alpha, omega-dicarboxylic acid (C7-C16); and omega-hydroxy long-chain fatty acids (C16-C22). Additionally, the lipophilic extractive was dominated by piceol, heptacosane, β-sitosterol, and fatty acids (C16-C28) from the studied hybrids. These findings could boost our integrative approach toward full valorization of willow bark.